1. Field of Invention
This invention relates to a semiconductor differential pressure measuring device which has excellent static pressure and temperature characteristics and is economical to produce and operate.
2. Description of Related Art
FIG. 1 shows an example of a conventional differential pressure measuring device, such as disclosed in Japanese Patent application No. S59-56137 (1984), wherein flanged 2 and 3 are assembled with close fit and fixed, such as by welding, to both sides of housing 1. Inlet port 5, used to apply high pressure fluid of pressure P.sub.H to be measured, and inlet port 4, used to apply low pressure fluid of pressure P.sub.L to be measured, are provide in flanges 2 and 3, respectively.
Housing 1 contains pressure measuring chamber 6 comprising center diaphragm 7 and silicon diaphragm 8, which are separately fixed to the wall of pressure measuring chamber 6, so that pressure measuring chamber 6 is divided into two by both diaphragms 7 and 8. On the walls of chamber 6, facing center diaphragm 7 are backplates 6A and 6B. The circumference of center diaphragm 7 is welded to housing 1. The entire silicon diaphragm 8 may comprise a single crystal silicon substrate.
Four strain gages 80 are formed by selectively diffusing impurities, such as boron, on one side of the silicon substrate, while the other side of the substrate is machined and etched to form the complete side as a concave diaphragm. Four strain gages 80 operate in such a manner that two of the four gages are subjected to tension and the other two are subjected to compression when the silicon diaphragm is deflected due to differential pressure .DELTA.P. These four strain gages are connected together to form a Wheatstone bridge, and the resistance change is detected as a change in the differential pressure .DELTA.P.
Leads 81 have one end thereof connected to strain gage 80. The other end of leads 81 are connected to hermetic terminals 82. Support 9 is provided with the hermetic terminals. Silicon diaphragm 8 is adhered to the end face of pressure measuring chamber 6 of support 9 by a suitable method, such as bonding with low melting point glass.
Pressure inlet cells 10 and 11 are formed between housing 1 and flange 2 and between housing 1 and flange 3, respectively. In pressure inlet cells 10,11, liquid blocking diaphragms 12 and 13 are provided, respectively, and on the walls of housing 1 facing liquid blocking diaphragms 12,13, backplates 10A and 11A, having similar shapes to liquid blocking diaphragms 12,13, are formed.
Spaces formed with liquid blocking diaphragms 12,13 and backplates 10A and 11A, respectively, are connected to pressure measuring chamber 6 through communicating holes 14,15. Spaces between liquid blocking diaphragms 12,13 are filled with liquids 101,102 such as silicone oil. The liquids reach the upper and lower faces of silicon diaphragm 8 through communicating holes 16,17. Fill liquids 101 and 102 are separated by center diaphragm 7 and silicon diaphragm 8, but the arrangement controls the placement of the liquids so that the two volumes of liquid are nearly equal.
If the pressure is provided at the high pressure side, the pressure which acts on liquid blocking diaphragm 13 is transmitted to silicon diaphragm 8 by fill liquid 102. On the other hand, if a pressure is provided at the low pressure side, the pressure which acts on liquid blocking diaphragm 12 is transmitted to silicon diaphragm 8 by fill liquid 101. Consequently, silicon diaphragm 8 is deflected in accordance with the pressure difference between the high pressure and low pressure. This deflection is detected by strain gage 80 and measurement of differential pressure is thus effected. However, such a device is adversely affected by static pressure which causes static pressure error.
In order to compensate for the static pressure error, diffusion strain gages G.sub.d and G.sub.s are provided on thin wall part 8a and thick wall part 8b of silicon diaphragm 8, as shown in FIG. 2. Strain gage G.sub.d on thin wall part 8a detects deformation of thin wall part 8a due to the differential pressure .DELTA.P. Strain gage G.sub.s on thick wall part 8b detects the value of static pressure Sp by sensing the deformation in silicon diaphragm 8 caused by the difference in deformation between silicon diaphragms 8 and support 9, when static pressure S.sub.p is applied to the entire detector.
The output change of strain gages G.sub.d and G.sub.s due to differential pressure .DELTA.P is as shown in FIG. 3. The output change in strain gages G.sub.d and G.sub.s due to static pressure S.sub.p is as shown in FIG. 4. That is to say, when differential pressure .DELTA.P is applied, the output of strain gages G.sub.d and G.sub.s varies.
For this reason, applied differential pressure .DELTA.P is determined by using the following equation (1) ##EQU1## In this case, since the outputs of strain gages G.sub.d and G.sub.s are closely correlated, separation of outputs is not sufficient. Thus, there are problems that make correction computations of higher orders (n,m) necessary, and accuracy after correction is still not sufficient.